This invention relates to novel storable and curable epoxy resin compositions, to novel curing agents incorporated into said compositions, to processes for making and curing the aforesaid compositions, a process for making the aforesaid curing agents, and to articles prepared from the aforesaid compositions.
Imidazoles are widely used as curing agents for epoxy resins because products cured therewith generally exhibit highly desirable physical and chemical properties. However, like most tertiary-nitrogen-containing curing agents, imidazoles react very rapidly with epoxy resin systems, even at room temperatures. Illustrative of this fact is the information compiled in the following Table A wherein a number of imidazole compounds are evaluated with respect to the physical properties they contribute to an epoxy resin system cured with the imidazoles, with respect to their base strength, and with respect to their cure processing characteristics.
For this evaluation, all imidazoles were ground to 325 mesh before being stirred into the epoxy resin which is a liquid diglycidyl ether of bisphenol A having an epoxide equivalent weight of about 180 to 190 grams.
TABLE A __________________________________________________________________________ CURE TIME AND SHELF LIFE OF EPOXY RESIN COMPRISING IMIDAZOLE COMPOUNDS AS CURING AGENTS Melting Boiling Curing Agent Point Point Base Strength lbs Curing Agent Cure Time.sup.1 Gel Time.sup.2 .degree. C .degree. C pK.sub.A per 100 lbs Resin at 140.degree. C at 25.degree. C __________________________________________________________________________ Compound Imidazole 90 256 6.9 2.5 2 min. 3 days 1 Methyl- -6 198 7.2 3 1.5 1 2 Methyl- 146 265 7.9 3 1.5 2 2 Ethyl- 86 265 -- 3.5 1.5 3 2,4 Dimethyl- 87 265 -- 3.5 1.5 3 1 benzyl, 2 Methyl- 45 240 -- 6.3 2 3 2 Ethyl, 4 Methyl- 44 293 -- 4.0 1.5 3 2 Phenyl- 145 340 6.4 5.3 2 3 Benzimidazole 170 -- 5.5 6.0 3.5 6 2 Phenyl-2-Imidazoline 103 -- -- 6.0 &gt;10 6 __________________________________________________________________________ .sup.1 Time for a 2 gram drop of the mixture to harden on a plate heated at 140.degree. C. .sup.2 Days for a 100 gram mixture to thicken to a gel at room temperature.
All of these imidazoles, irrespective of their melting or boiling point, cause rapid cure of a liquid epoxy system at temperatures on the order of 140.degree. C. at low weight ratios of imidazole to epoxy resin. However, because they cause gelation at 25.degree. C. none of these imidazole compounds can be used to provide a one-part system wherein the imidazole is intimately blended as a powder or as a liquid to provide a composition which is useful for an extended period of time. In general, such mixtures as shown in Table A gel within three to six days at only 25.degree. C. and therefore are not suitable for use as storeable "one-package" unitary compositions.
Therefore, resin systems utilizing such curing agents are not adapted for storage as unitary systems comprising both the imidazole curing agent and an epoxy resin to be cured thereby. Unitary systems are highly desirable because they allow manufacturers and consumers of epoxy compositions to avoid more complex packaging means, to avoid the additional mixing step necessary before a separately packaged epoxy resin can be converted into a cured material, and to avoid the probability that an incorrect amount of curing agent will be added to the resin by the ultimate consumer.
Previous attempts to solve these problems by producing such unitary systems have not been entirely satisfactory. This has been because of the highly catalytic nature of tertiary nitrogens in imidazole compounds. The problem has been to suppress this catalytic action without permanently destroying the thermal and chemical reactivity required for an eventual catalytic cure.
It is not sufficient to select a solid imidazole compound, or a solid tertiary amine compound having a relatively high melting point; such compounds do not provide satisfactory shelf life on the subsequent incorporation of these powdered solids into epoxy-containing formulations. For example, 2-methyl imidazole and other imidazoles, shown in Table I have relatively high melting points, &gt; 140.degree. C., yet cause a typical liquid epoxy resin (described herein below as Epoxy A) to gel overnight at 35.degree. C. This effect was noted when the imidazole was ground to 200 mesh and incorporated at a weight ratio of 1:50 into the resin. Likewise, triethylene diamine, a solid tertiary diamine melting about 165.degree. C. causes the gelling of such resins as Epoxy A when incorporated at a level of about 10% and left standing therein at 35.degree. C. for 2 days.
Moreover, previous attempts to provide epoxy curing materials useful in curing processes carried out at relatively low temperatures, e.g., room temperature and slightly above, have involved the preparation of salts of imidazole compounds with such acids as acetic acid and lactic acid. Such compounds are disclosed in U.S. Pat. Nos. 3,356,645 and 3,329,652. In the former patent, simple salts of imidazoles are formed. Such materials, on incorporation in epoxy resins, generally produce compositions of relatively short shelf life. For example, in U.S. Pat. No. 3,356,645 a pot life of only 5 days is obtained with 2-ethyl, 4-methyl imidazole acetate salts. In U.S. Pat. No. 3,329,652 these same salts are disclosed to form useful epoxy-curing initiators in anhydride-cured systems.
Until the instant invention, there has been no really effective epoxy curing agent which would be suitably used in unitary compositions such as liquid adhesives and potting compounds, solid fluid-bed coating compositions and other such materials. For real commercial value, a curable epoxy resin composition incorporating such curing agents should remain at workable viscosities more than 30 days at 25.degree. C. and cure within 10 minutes at about 140.degree. C., and such compositions can be prepared in accordance with the present invention.
Consequently, the principal object of the instant invention is to provide novel imidazole-type curing agents which are useful in making unitary curable epoxy resin compositions which satisfy the above criterion.
Another object of the invention is to provide imidazole-type curing agents which alone or together with other curing agents, can activate a rapid, uniform cure of epoxy resin compositions at relatively low temperatures and yet be extraordinarily resistant to gelling at room temperature for weeks or even months.